Squamous cell carcinomas of the head and neck (HNSCC) are among the most immunosuppressive human tumors. Tumor-induced and therapy-induced immune suppression may be responsible for disease recurrence seen in over 60% of SCCHN patients receiving curative therapies. Our data indicate that adenosine and PGE2 are among the major immunosuppressive factors in HNSCC. Not only the tumor but also regulatory T cells (Treg) produce these factors, and adaptive Treg accumulate in the tumor microenvironment during tumor progression. Their frequency and function increase after chemoradiation (CRT) and remain elevated for months, potentially contributing to tumor recurrence. In this proposal, we test the hypothesis that restoration of anti-tumor immunity in the microenvironment of HNSCC can be achieved in vitro and in vivo by down-regulating Treg and enhancing Teffector cell (Teff) functions using pharmacologic agents which target the adenosine/PGE2 pathway. In three aims, we propose to: (1) conduct a prospective non-therapeutic longitudinal study in HNSCC patients treated with surgery and CRT to ask whether persistent immune suppression and enhanced activity of the adenosine/PGE2 pathway after CRT contribute to the tumor recurrence; (2) study in vitro effects of the pharmacological blockade of the adenosine/PGE2 pathway on Treg-mediated suppression and potentially concomitant restoration of anti-tumor functions of Teff; and (3) show that metabolic silencing of the adenylyl cyclase and stimulation of phosphodiesterase activity in Teff in vivo promotes anti-tumor immune responses to a multi-epitope vaccine, induces tumor rejection and prolongs survival of 4NQO mice with oral carcinoma. These pre-clinical studies are designed to demonstrate that persistent accumulations of Treg in the human tumor microenvironment are associated with tumor progression and that the pharmacological blockade of a common immunosuppressive pathway alone or in combination with conventional cancer therapy silences Treg, restores anti-tumor immunity and inhibits tumor growth. The potential of this new combinatorial therapy for re-establishing effective anti-tumor immunity following CRT in HNSCC is expected to provide the rationale for its translation to the clinic.